This invention relates to a light-emitting semiconductor device, or light-emitting diode (LED) according to more common parlance, and more particularly to high-efficiency light-emitting devices having active layers made from semiconducting chemical compounds such for example as aluminum gallium arsenide (AlGaAs), aluminum gallium indium phosphide (AlGaInP), gallium nitride (GaN), and derivatives thereof. The invention also concerns a method of making such light-emitting semiconductor devices.
A typical conventional light-emitting semiconductor device has a substrate of gallium arsenide (GaAs) on which there are successively grown a set of semiconductor layers including light-generating active layers. These active and associated semiconductor layers, hereinafter collectively referred to as the light-generating semiconductor layers, are each composed principally from AlGaInP. Comparatively well lattice matched with the GaAs substrate, the AlGaInP-based light-generating semiconductor layers are favorable in crystallinity.
There does, however, exist a crucial drawback to the GaAs substrate: It is highly absorptive of the light of the particular wavelength range produced by the AlGaInP-based active layers. Much of the light radiated toward the GaAs substrate from the active layers was therefore wasted, running counter to the objective of making the light-emitting device as high in efficiency as could be desired.
A known remedy to this problem was to remove the GaAs substrate from under the light-generating semiconductor layers after epitaxially growing these layers thereon. A transparent baseplate of gallium phosphide (GaP) or the like, different from the removed substrate which had been used for epitaxial growth of the light-generating semiconductor layers thereon, was then bonded to the underside of these semiconductor layers. Then a reflective electrode was formed under the baseplate. This remedy proved unsatisfactory, however, as the light-generating semiconductor layers and the transparent baseplate gave rise to high electrical resistance at the interface therebetween. This resistance made the forward voltage between the anode and cathode of the light-emitting device inconveniently high.
A solution to this weakness of the known remedy is found in Japanese Unexamined Patent Publication No. 2002-217450 filed by the applicant of the instant U.S. application. This prior patent application teaches the creation of a thin, open-worked layer of gold—germanium—gallium (Au—Ge—Ga) alloy on the underside of the light-generating semiconductor layers. The open-worked Au—Ge—Ga alloy layer, as well as those surface parts of the overlying light-generating semiconductor layers which are left exposed by this open-worked alloy layer, is then covered with a layer of aluminum or like reflective metal. To this reflective metal layer is then bonded a baseplate, or mechanical support, of electrically conductive silicon or like material.
The Au—Ge—Ga alloy layer is known to make favorable ohmic contact with AlGaInP-based light-generating semiconductor layers, so that it can reduce the forward voltage between anode and cathode. The efficiency of light emission is also enhanced as the reflective metal layer reflects the light that has been radiated toward the baseplate.
However, this second recited prior art device also proved to have its own weaknesses. One of these weaknesses arose in the course of the manufacturing process of the device, which involved several heat treatments. Undesired reactions took place as a result of such heat treatments between the reflective metal layer and the neighboring parts of the light-generating semiconductor layers. The result was a diminution of reflectivity at their interface. The light-emitting devices actually manufactured according to this prior art were therefore not necessarily so high in efficiency as had been expected.